The invention relates generally to radio communication and more specifically to a detector for radio signals which have been transmitted through a multipath medium.
In looking at the problem of communicating with aerospace vehicles in the 100 Mhz.-2000 Mhz. range, it becomes obvious that one of the fundamental challenges is that of reflection from the earth's surface, or "multipath". In approaching the problem, the system engineer's first inclination is, perhaps, to try to "evade" the multipath. For communication with a single vehicle, such as space shuttle, directivity in the ground antenna can be employed to eliminate illumination of the surface for all but very low elevation angles of the vehicle, provided a tracking antenna may be employed. For communication with limited numbers of vehicles, use may be made of short pulses with long waiting times between pulses to "beat the reflection" and "wait out the echoes." A variation of this scheme is "frequency-hopping" where the signal does not remain at any one frequency long enough for the reflection to arrive at that frequency. Unfortunately, waiting between pulses places a restriction on information rate and frequency-hopping presents an interference problem for the many vehicle cases.
It seems reasonable to assume that aerospace data-links will employ digital signalling. From the standpoints of economy and maintainability of hardware, simple modulations, such as binary phase-shift-keying or binary frequency-shift-keying, seem indicated. Within the framework of simple modulation, and noting the restrictions caused by using modulation to evade the multipath reflection, it seems that one should deal with the detection process in attempting to combat multipath. By this is meant using modern detection and estimation methods in receiving given digital modulations in the presence of multipath interference. Multipath-adaptive detection seems especially attractive. It is therefore the primary purpose of this invention to provide a digital detector for radio signals that combats multipath interference.
The prior art for continuous detection of digital signals in multipath, using analog circuitry is not of direct concern to this disclosure. The prior art for sampled-data detection of digital signals in multipath is of direct concern. A sampled-data detector, using digital techniques, was published in IRE Transactions on Information Theory, Vol. IT-6, No. 3, pp. 361-366, June 1960. However, that detector was not recursive and required storing all the data samples, simultaneously, which were taken during the basic message symbol interval of length T. When using many samples per symbol interval, the data storage requirements for the nonrecursive detector are so large as to render the digital implementation of the detector impractical. The invention disclosed herein is a practical, fully recursive, digital detector for digital signals in multipath.